The present invention relates to a technique for personal authentication utilizing iris images, and more particularly, it relates to a technique to improve the accuracy in iris authentication in outdoor light such as sunlight.
Recently, a technique for personal authentication utilizing iris images has started to be employed for management of entrances/exits of an important institution, authentication of a user of an ATM (Automated Teller Machine) of a bank or the like, authentication for logging in a PC and the like. In general, the iris authentication is performed through the following steps:
1. An iris image of a person is captured with a near-infrared illumination (ex. LED);
2. The iris image is analyzed to extract an iris code; and
3. The extracted iris code is compared with a previously registered iris code, and when a difference (distance) between these codes is not more than a threshold value, the person is authenticated.
In this case, the near-infrared light is used for lighting because it is not perceptible for man and hence does not dazzle the person to be authenticated. In addition, most of the people on the earth have dark brown irises (although some people have blue or gray irises, they are globally minor), and the iris pattern of a dark brown iris is difficult to visually recognize in visible light but its image can be captured with high contrast in the near-infrared light. The iris pattern of a blue or gray iris can be also captured in the near-infrared light.
As described above, the iris authentication has been started to be employed for the management of entrances/exits and the authentication of users of ATMs and for logging in PCs, and in most cases, the iris authentication is employed from a premise that it is performed indoor with a small quantity of near-infrared component of outdoor light. In the case where the iris authentication is performed in outdoor light, such as the sunlight, including a large quantity of near-infrared component, near-infrared light from the sun and the like and near-infrared light reflected by objects irradiated by the sun and the like are widely reflected in the eye. Also, in the case where the face is directly exposed to the sunlight, the shadow of the eyelid or eyelashes may be cast over the iris region. These factors can increase a false rejection rate (FRR). However, it is almost impossible that an obtained iris pattern accidentally resembles an iris pattern of another person due to such reflection and shadow of the eyelid or eyelashes. Therefore, there is little possibility of increase of a false acceptance rate (FAR).
Mostly, the facilities for an ATM is located in the vicinity of the entrance of a building facing on a street. Methods for preventing the influence of the outdoor light are described in patent documents 1 and 2. According to the patent document 1, reflection of the extraneous light is prevented as follows: First polarizing means is provided outside a lens of a camera for capturing an iris image, second polarizing means is provided between an iris authentication apparatus and the source of extraneous light (for example, on a window), and the first and second polarizing means have different polarization directions. Alternatively, according to the patent document 2, the reflection of the extraneous light on an eye is prevented by providing, in a position from which the extraneous light enters the eye (for example, on a window), a non-visible light non-penetrating filter for reflecting or absorbing a non-visible light component lighting the eye out of frequency components of the extraneous light.
As alternative means for preventing the reflection, shielding means such as an eye cup is provided in front of a camera, and a person to be authenticated (hereinafter simply referred to as a user) looks into the shielding means for capturing an image of his/her iris.
Furthermore, patent documents 3, 4, 5 and 6 describe methods for dealing with reflection of lighting means itself used for lighting an eye, which is not the outdoor light.
According to the patent document 3, the head of a user is expected to move while a plurality of iris images are being captured, and the plural iris images are used for reducing the influence of the reflection. First, one image and a registered image are subjected to matching, and a consistent portion therebetween is added to a matching image. Thereafter, the other images are also subjected to the matching with the registered image, so as to successively add consistent portions to the matching image. The ultimate matching image thus formed is compared with the registered image for the authentication.
In all the techniques disclosed in the patent documents 4, 5 and 6, a plurality of lighting means installed in different positions are used. Specifically, an eye is lighted with the plural lighting means at different timings, so as to capture a plurality of iris images respectively at the different lighting timings. A plurality of iris images (or features or comparison results) in which the positions of the reflection of the lighting means are respectively different are synthesized, so as to generate an iris image (or a feature or comparison result) that is free from the influence of the reflection.
Patent document 1: Japanese Laid-Open Patent Publication No. 10-21392
Patent document 2: Japanese Laid-Open Patent Publication No. 2000-185032
Patent document 3: Japanese Laid-Open Patent Publication No. 9-212644
Patent document 4: Japanese Laid-Open Patent Publication No. 10-162146
Patent document 5: Japanese Laid-Open Patent Publication No. 11-203478
Patent document 6: Japanese Laid-Open Patent Publication No. 2001-167252
The aforementioned conventional techniques have, however, the following disadvantages:
The techniques disclosed in the patent documents 1 and 2 are described from a premise of the facilities such as an ATM, and hence, both the techniques need a large-scale apparatuses and need a large cost for practice. Therefore, these techniques are disadvantageously limited in the applicable use.
Also, in the method in which the shielding means such as an eye cup is provided in front of a camera and a user looks into the shielding means for capturing the image of an iris, an advantage of the iris authentication, that is, contactless authentication, is spoiled. Therefore, this method is not preferable from the viewpoint of sanitation and user interface.
Furthermore, the patent document 3 originally describes the countermeasure against not the reflection of the extraneous light but the reflection of the lighting means equipped on the authentication apparatus. Moreover, although the head of a user is expected to naturally move, when the head does not move, the influence of the reflection cannot be reduced and hence the effect cannot be attained. Furthermore, even if the head moves, the user stands in front of the apparatus for the authentication and hence dare not turn his/her eyes largely off the apparatus. Accordingly, there is little possibility that the position of the reflection is changed.
Also, the methods described in the patent documents 4, 5 and 6 using plural lighting means are useful as the countermeasure against the reflection of the lighting means equipped on the authentication apparatus but are useless against the reflection in the outdoor light.